This invention relates to light modulating devices, and is concerned more particularly, but not exclusively, with liquid crystal display and optical shutter devices including spatial light modulators.
It should be understood that the term xe2x80x9clight modulating devicesxe2x80x9d is used in this specification to encompass both light transmissive modulators, such as diffractive spatial modulators, and light emissive modulators, such as conventional liquid crystal displays.
Liquid crystal devices are commonly used for displaying alphanumeric information and/or graphic images. Furthermore liquid crystal devices are also used as optical shutters, for example in printers. Such liquid crystal devices comprise a matrix of individually addressable modulating elements which can be designed to produce not only black and white, but also intermediate tones, or colour hue variations in devices in which colour filters are used. The so-called greyscale response of such a device may be produced in a number of ways.
In a spatial dither (SD) technique each modulating element is divided into two or more separately addressable subelements which are addressable by different combinations of switching signals in order to produce different overall levels of grey. For example, in the simple case of an element comprising two equal sized subelements each of which is switchable between a white state and a black state, three grey levels (including white and black) will be obtainable corresponding to both subelements being switched to the white state, both subelements being switched to the black state, and either subelement being in the white state while the other subelement is in the black state. Since both subelements are of the same size, the same grey level will be obtained regardless of which of the subelements is in the white state and which is in the black state, so that the switching circuit must be designed to take account of this level of redundancy. It is also possible for the subelements to be of different sizes which will have the effect that different grey levels will be produced depending on which of the two subelements is in the white state and which is in the black state. However, a limit to the number of subelements which can be provided in practice is imposed by the fact that separate conductive tracks are required for supplying the switching signals to the subelements and the number of such tracks which can be accommodated is limited by space constraints.
In a temporal dither (TD) technique at least part of each modulating element is addressable by different time modulated signals in order to produce different overall levels of grey. For example, in a simple case in which an element is addressable by two subframes of equal duration, the element may be arranged to be in the white state when it is addressed so as to be xe2x80x9conxe2x80x9d in both subframes, and the element may be arranged to be in the black state when it is addressed so as to be xe2x80x9coffxe2x80x9d in both subframes. Furthermore the element may be in an intermediate grey state when it is addressed so as to be xe2x80x9conxe2x80x9d in one subframe and xe2x80x9coffxe2x80x9d in the other subframe. The frame rate should be greater than the frequency at which the dither is observable as flickering. Furthermore it is possible to combine such a temporal dither technique with spatial dither by addressing one or more of the subelements in a spatial dither arrangement by different time modulated signals. This allows an increased range of grey levels to be produced at the cost of increased circuit complexity.
In many applications, and particularly in display devices for displaying moving graphic images, there is a requirement for a large number of suitably spaced grey levels to be generated, with minimum (and preferably no) redundancy of grey levels. Usually the grey levels are linearly spaced as far as possible. To this end the elements may be binary weighted, for example by dividing each element into subelements having surface areas in the ratio 1:2:4 in a SD technique or by addressing of each element with frames having durations in the ratio 1:2:4 in a TD technique. European Patent Publication No. 0261901 A2 discloses a method of maximising the number of grey levels that can be obtained from a certain number of binary temporal divisions of the addressing frame by dividing the addressed rows of the display matrix into groups and addressing the groups sequentially.
Furthermore U.S. Pat. No. 4,712,877 discloses a method of producing discrete grey states within a pixel of a ferroelectric liquid crystal display device by a technique called multi-threshold modulation (MTM), generally by variation of the electric field over the pixel area. For example the liquid crystal thickness may be varied over the pixel area in steps. This method may be combined with dither techniques in order to produce a large number of grey levels, although in practice it is difficult to address more than a few MTM grey states.
There are a number of inherent physical problems encountered in ferroelectric liquid crystal display devices which result in finite errors in the analogue grey states, and which can accordingly result in unpredictable variation of grey levels with time and/or over the display area. Such problems are discussed in P. Maltese, xe2x80x9cAdvances and problems in the development of ferroelectric liquid crystal displaysxe2x80x9d, Mol. Cryst. Liq. Cryst. 1992, Vol. 215, pp. 57-72, as well as in K-F. Reinhard, xe2x80x9cAddressing of ferroelectric liquid crystal matrices and electrooptical characterisationxe2x80x9d, Ferroelectrics 1991, Vol. 113, pp. 405-417. As is well known, analogue grey states are highly temperature dependent, and the latter reference gives an example in which the display temperature should be uniform to 0.2 degrees if 16 grey levels are required. Both references indicate that the use of thin film transistors for the drive circuitry is advantageous to achieve analogue grey states in such devices.
British Patent Application No. 9603506.8 and Japanese Patent Publications Nos. 27719/1993 and 27720/1993 describe techniques for reducing the error in a 50% analogue grey state to substantially zero by dividing each row (strobe) electrode into two subrows and simultaneously addressing the two subrows such that any local temperature variation has opposite effects in the two subrows tending to cancel the temperature dependence of the grey state for each row. Such a technique allows a substantially error free half (50%) analogue grey state to be obtained. Japanese Patent Application No. 9-72198/1997 describes a technique for obtaining such a substantially error free half state which uses an interlace technique to avoid the need to introduce extra subrows. The term xe2x80x9csubstantially error freexe2x80x9d should be interpreted in this context as meaning that the error associated with such a state is small by comparison with the errors associated with analogue intermediate grey states produced by conventional means.
A digital error occurs when more significant bits of a digital dither arrangement change between successive frames (temporal dither) of adjacent pixels (spatial dither). It is an objective of the present invention to ameliorate this disadvantage.
According to the present invention, there is provided a light modulating device comprising an addressable matrix of modulating elements, and addressing means for selectively addressing each element in order to vary the transmission level of the element relative to the transmission levels of other elements, the addressing means including spatial and/or temporal dither means for addressing separately addressable spatial bits of each element with different combinations of spatial dither signals and/or for addressing at least part of each element with different combinations of temporal dither signals applied to separately addressable temporal bits corresponding to subframes of different periods to produce a plurality of different transmission levels, and state selection means for switching at least a part of each element between different states corresponding to different transmission levels by means of ON and OFF switching signals, wherein the state selection means includes means responsive to at least one transition in grey level between adjacent and/or successive elements which transition involves changing the state of a bit between a first state and a second state for providing at least one intermediate element for which the light transmission level of that bit is intermediate that of the first state and the second state.
Thus at least one problematic transition between grey levels is identified and ameliorated.
While the invention may respond to only one change in grey level, preferably it responds to at least those changes that cause a digital error. Possibly, it responds to all of the possible changes in grey scale.
In a preferred embodiment, the compensating signal is provided by way of an analogue level in the most significant digital bit. This level could be half the transmission level or at some other level. The level may advantageously depend upon whether the most significant bit is changing from dark to light or vice versa.
Further preferred features of the invention will be apparent from the detailed description of preferred embodiments which follows.